(1) Field of the Invention
The present invention relates to an image forming apparatus such as an electrostatic photocopier, a printer and the like and more particularly relates to an image forming apparatus equipped with a fan for inhibiting temperature rise inside the apparatus.
(2) Description of the Prior Art
Conventional copiers use a halogen lamp as a light source for an exposure device which illuminates an original at the time of copying. This is because the halogen lamp is highly efficient as compared to a typical incandescent lamp.
The halogen lamp, however, gives out a great amount of heat, therefore if a prolonged period of use such as consecutive copying operations are made, the surface of the original table glass near the exposure device is heated to a great extent. In order to prevent a user from being burned when the user touches the original table glass, some domestic and foreign safety standards have been established which limit the surface temperature of the original table glass to a regulated level or less. To meet the standards, a cooling fan is provided for a copier of this kind to cool by force the original table glass as well as the vicinity of the exposure device. As to the inside of the copying machine, heat from the heater for the fixing unit as well as heat from the exposure device and other devices elevates the temperature inside the machine. Further, ozone gas etc., generated from the charging device affects the photoreceptor drum, the developing unit and the like to thereby degrade the quality of copy. To prevent this, a ventilating fan is provided in order to lower the inside temperature of the machine and to exhaust air inside the machine.
These fans are driven in response to signals from a controller provided in the copier. That is, when the user inputs a number for copies through a control section 1 and activates the copying operation, a micro-computer 3 in a controller 2 outputs three levels of signals to a fan controlling circuit 4 in accordance with the designated number of copies, as shown in FIG. 1. More specifically, the rotating rate of a fan 5 is varied during the copying operation so that the cooling operation may be effected by three levels, that is, strong blow, middle blow and weak blow in association with the designated number of copies. When the copier is disengaged, such blow cooling is not needed, so that the fan 5 is reduced in its rotating rate or stopped in order to decrease noises therefrom and save the energy. In the disengaged mode, if, even with only the fan 5 stopped, temperature control of the heater in the fixing unit is done as usual, the power consumption is hardly reduced, therefore little energy can be saved. Since such a configuration has little effect on saving energy and it is impossible for the system to lower the temperature inside the machine, the temperature of the heater in the fixing unit should also be lowered during the disengaged mode as compared to the usual temperature at which fixing is allowed. Actually, this was done in the conventional configuration. Meanwhile, the configuration in FIG. 1 includes a main switch 5, an a.c. power supply 7, a power source circuit 8, a machine controlling circuit 9 for controlling various parts of the copier in response to output signals from a micro computer.
Several methods which use fan control of this kind, that is, the control based on the designated number of copies, have been proposed in, for example, Japanese Patent Application Laid-Open publications Sho 61 No.219,966, Hei 1 No.209,458, Hei 1 No.214,836 and Hei 2 No.103,060. Specifically, these publications disclose copiers in which temperature rise inside the machine is prevented and noises are reduced by switching a fan for cooling an exposure lamp between strong and weak intensity modes by turning on and off a fan and by varying the operating time of a fan with reference to the designated number of copies at copying.
Nevertheless, since the characteristics of temperature rise of the original table glass and the machine-inside of the copier vary depending upon not only the number of copies, but also upon specific copying operations such as the sheet size, magnification, copying density and the like, the aforementioned methods, that is, the switching of blow intensity of the fan, the on-and-off switching of the fan, the switching of operating time of the fan in accordance with the designated number of copies are not enough to regulate actual temperature rise or variation of the original table glass and the machine-inside of a copier and therefore the copier still suffers from insufficiency of cooling or excessive cooling. Particularly, if the copier-inside is excessively cooled, the heater temperature in the fixing unit should become high, therefore power consumption becomes large due to the heat loss, and consequently, efficient operation of the copier and comfortable setting environment are deteriorated. Besides, in the copier in which the fan is turned on and off, a supplied blowing amount of air is determined by the specifications of the fan used, therefore, the noise characteristics and the maximum air-blowing amount (required air-blowing amount) are also determined just by the capacity proper to the fan used (without any maximum limit). In other words, in this conventional configuration, no efficiency of the fan was taken into account for cooling and lowering noises.
To solve the above problems, Japanese Patent Application Laid-Open publications Hei 3 No.144,627, Hei 4 No.238,339 and Hei 4 No.372,941 disclose copiers and the like in which the air-blowing amount of a fan for cooling an exposure lamp is switched between strong and weak intensity modes or among strong, medium and weak intensity mode with reference to designated copying specifications such as the number of copies, recording sheet size, magnification, exposure lamp intensity, scan length etc. In one word, these disclosures intend to control the operation of fan by assuming the temperature rise according to designated copying specifications and regulating the operation of the fan based on the assumption.
Since, even such copiers, the adjustment of air-blowing amount of the fan is merely made between two modes (strong and weak intensity modes) or three modes (strong, medium and weak modes), the conventional cooling control system is not enough efficient to suffice the recent demands such as of energy saving as well as of restricting the generation of ozone gas which has adverse influence on the human body.
Further, although the conventional system assumes a temperature rise based on the copying specifications, this assumption is made for only a future copying operation without taking the past use of the machine into consideration. Therefore, if, for example, a second copying operation is effected before the heat of the original table glass and the machine-inside due to a first copying operation is enough cooled down, the temperature of the original table glass and the machine-inside is elevated from a high-temperature state to a still higher-temperature state. Accordingly, the actual temperature differs from the temperature expected based on the second copying specifications and consequently, insufficiency of cooling occurs. That is, when two copying operations were made with a small interval, the conventional configuration could neither cool down the machine properly nor efficiently while it was impossible to realize reduced noises and saved energy.